The invention relates to a method and a measurement device for determining the temperature of an away-facing surface of an object and to a use of the method and a use of the measurement device. A corresponding method and a corresponding device for temperature determination by ultrasound may be gathered from JP 2003042857 A.
Turbomachines, such as, for example, steam or gas turbines, are used as thermal engines in industry in order to convert an energy stored in a gas stream into a mechanical energy, particularly into a rotational movement. Furthermore, turbomachines, such as compressors, by which mechanical energy can be supplied to a gas stream, also come under consideration. In order, in gas turbines, to achieve as high an overall efficiency as possible in terms of energy utilization, the selected gas inlet temperatures from the combustion chamber into the flow duct of the gas turbine are as high as possible. For example, the gas inlet temperatures lie above 1000° C.
This makes it necessary, under these high physical loads, for the turbomachine to be kept under observation during operation. In this case, in particular, temperature measurement within the turbomachine delivers important information on the state of the turbomachine. For this purpose, as a rule, measuring probes mounted in the turbomachine are used, the signal and supply lines of which lead outward through the wall of the turbomachine by leadthroughs. A large number of temperature measurement points therefore require a large number of leadthroughs and seals. Under the high physical loads, these always constitute fault sources which must be avoided as far as possible in order to ensure that the turbomachine operates reliably.
JP 2003042857 A specifies a method and a device, by which the temperature of a wall surface which faces away from the device and is in contact with a liquid and, consequently, the temperature of the liquid can be measured by ultrasound. In this case, ultrasonic waves are irradiated through the wall surface facing the device into the wall and are reflected on the surface facing away. The ratio of the amplitudes of the ultrasonic waves before and after reflection on the away-facing surface which is in contact with the liquid depends in this case on the acoustic reflection factor of the surface, which is determined, in turn, by the acoustic impedance of the liquid. Since the acoustic impedance of the liquid is temperature-dependent, the temperature of the liquid can ultimately be determined from the determination and evaluation of the reflection-induced amplitude decrease of the ultrasonic radiation. To determine the amplitude decrease, the amplitude peak values are measured, although these are overlaid with interference signals and noise. It is therefore scarcely possible to determine the amplitude decrease exactly after only one reflection. For this reason, the amplitude decrease of ultrasonic waves which are multiply reflected from the two surfaces standing opposite one another is investigated. However, where relatively thick walls are concerned, this cannot be carried out, since the amplitudes of the ultrasonic waves, which also experience damping when they run through the wall, are excessively attenuated after multiple reflection and are therefore no longer measurable. Moreover, the method and the device require a liquid which is in contact with the wall and by which the reflection factor can be noticeably influenced. By contrast, where gases are concerned, a temperature-dependent change in the reflection factor is no longer measurable, since, because of the very high reflection factor of approximately 1, its slight change is no longer detectable unequivocally, particularly in the background interference.